The seasonal and diurnal dynamics of methane flux at a created urban wetland

Wetlands provide important ecosystem services and store carbon dioxide but are also an important global source of methane – a potent greenhouse gas. In order to understand the dynamics of methane emissions from a temperate reconstructed wetland, methane fluxes were measured continuously over 2 years using the eddy covariance method in the Olentangy River Wetlands Research Park, where a simultaneous chamber measurement campaign is ongoing. The advantage in eddy covariance relative to chamber-based measurements is that measurements are integrated over a large area and are continuous, whereas chamber measurements are point measurements and sporadic in time. Having continuous measurements year-round allows us to determine the temporal patterns of methane emissions. We find distinct diurnal and annual cycles of flux rates. Peak summertime emissions were 2–3 times higher than peak wintertime emissions; however, overall wintertime emissions where not negligible and accounted for roughly 36–38% of the total annual flux. During the day, fluxes peaked around noontime and were lower during the night than during the day. The average flux for any half hour during the 24 h cycle remained above zero with nighttime fluxes contributing a significant portion of the diurnal total. We conclude that the temporal dynamics of methane emissions over the seasonal and diurnal time scales should not be ignored when designing the sampling schedule for a field campaign and when extrapolating data obtained in intermittent methane sampling to create a long-term methane budget for temperate wetlands.